1. Field of the Invention
Embodiments of the present invention relate generally to DC-AC power conversion, and more particularly, to bidirectional DC-AC power conversion in uninterruptible power supplies.
2. Discussion of Related Art
Uninterruptible power supplies (UPS) are used to provide continuous power to devices, or loads, when the primary power source, or mains, fails. Typical loads include computer systems with critical power requirements, but other loads, such as heating/cooling/ventilation systems, lighting systems, and televisions may also be supported by a UPS. A UPS designed for home use may provide backup power for anywhere from about five minutes to several hours. A UPS unit typically contains one or more low-voltage batteries (e.g., 12 V, 24 V, or 48 V), depending on the size of the unit, as a power source when AC mains is unavailable. DC power provided by the battery is converted to AC power by an inverter, which in turn is provided to the load. A battery charger, which converts AC power to DC power, may be included in the UPS to charge the battery when AC mains is available to ensure that backup power will be available when needed. The UPS may also include a control unit for automatically managing the operation of the UPS and the power conversion functions.
Various techniques have been developed for DC-AC power conversion in a UPS. In one technique, a low-frequency (e.g., 50/60 Hz) transformer is used to step up the battery voltage to an output voltage, and also to provide isolation between the battery and the load. Low-frequency transformers are bulky, heavy, and have poor power conversion efficiency. In another technique, a high-frequency transformer is used in place of the low-frequency transformer. Switching devices in the power conversion circuit create, for example, either a sine wave or square wave output voltage. There may be two separate power conversion channels, one for the inverter and another for the charger, or a single, bi-directional power conversion channel. Such devices require many high-voltage switches, a large transformer, high-current-rated low-voltage switches to support the load reactive power and large current, and/or additional components which increase manufacturing costs and also suffer from low power conversion efficiency and low utilization rates. Also, the control complexity is high.